Consider injecting Buffer to window in standalone script

Currently `Buffer` can be obtained by `window.buffer.Buffer`. But many application and library use `Buffer` as a global variable. They offen use `Buffer.from()` not `buffer.Buffer.from()`.
I suggest that we can provide a `window.Buffer` in standalone script.
If there is something I can do for helping this work, just let me know. Thanks for your great work!